The present invention relates to a process and apparatus for the gasification of solid carbon or carbonaceous material with hot gases from the partial oxidation of gaseous, liquid and solid fuels, in particular to the gasification in an entrained bed facility of coal, biomass and organic residual substances, e.g., from the recovery of waste.
The field of application of the invention is the production of fuel gas, synthesis gas and reduction gas from these fuels.
The gasification of solid carbon by means of hot gases has been known since the introduction of the processes for the production of gas by partial oxidation in fixed bed and in fluid bed reactors.
During gasification in a fixed bed reactor, the hot gas containing carbon dioxide is produced by burning solid carbon upstream in the direction of flow of the gasification medium of a so-called reduction zone. The gas carries into the reduction zone the gasification medium of carbon dioxide and the enthalpy necessary for the endothermic gasification of carbon to carbon monoxide. The partial oxidation, on the one hand, and endothermic gasification of carbon, on the other hand, thus take place in sequence, at separate locations and at different temperatures during fixed bed gasification.
The specific aspect of the gasification of fuels in the stationary or circulating fluid (fluidized) bed, on the other hand, consists of partial oxidation and endothermic gasification of solid carbon taking place practically simultaneously and at the same location, in an approximately isothermal manner.
Published patent specification WO95/21903 (corresponding to U.S. Pat. No. 5,849,050, the entire contents of which are incorporated herein by reference) discloses a method for the endothermic gasification of solid carbon with hot gas from partial oxidation in an entrained bed facility which is referred to in practice as chemical quenching. The basic principle of this process involves mixing solid carbon in the form of coal or coke from the degasification of fuels into a hot stream of gas from partial oxidation having a temperature of more than 1,200° C. and containing carbon dioxide and steam. The carbon reacts with the gas components of carbon dioxide and steam to form carbon monoxide and/or carbon monoxide and steam, by making use of the physical enthalpy of the hot gas, i.e., part of the physical high temperature enthalpy of the gas is reconverted by endothermic chemical reactions into chemical enthalpy. As a result of this measure, the calorific value of the gas increases as a result of which the degree of effectiveness of the conversion of the process is improved in comparison with those processes which merely make physical use of the physical enthalpy of the gas. During the practical application of the process disclosed in WO95/21903, it became apparent to the present inventors that the effectiveness of the endothermic gasification of solid carbon depends markedly on the method of operation of the process stages downstream and upstream, on the solid carbon charge of the hot gas and on the relative speed between gas and carbon.
In accordance with published patent specification DE 198 07 988 (corresponding to Canadian Patent No. 2,306,889, the entire contents of which are incorporated herein by reference) and similar devices, the thermal stage of processing the fuel, preferably biomass, into a tar-containing degasification gas and a tar-free coke produces a specific limited amount of coke, mainly as a result of the content of volatiles of the fuel and the heat requirement of the thermal recovery process. This coke is ground to a pulverized fuel that is suitable for pneumatic conveying, with a grain size of preferably <100 μm.
In the device according to DE 197 47 324 that is designed for implementing the process of patent specification WO95/21903, the tar-containing degasification gas is partially burned above the ash melting point with an oxygen-containing gasification medium in a combustion chamber, together with the residual coke obtained during dedusting of the gasification gas, in such a way that a hot, tar-free gasification medium containing not only CO and H2 but also CO2 and H2O is obtained. The fuel ash contained in the residual coke is melted during this process.
In accordance with DE 197 47 324, the hot gasification medium flows from the combustion chamber, together with the liquid slag, in the form of an immersion stream into the part of the entrained bed reactor arranged below the combustion chamber, in which reactor the endothermic reactions take place. This will be referred to as an endothermic entrained bed reactor in the following disclosure.
The finely ground coke dust is blown pneumatically via lances and nozzles into the immersion stream and, as a result of chemical quenching, leads to cooling of the gas and to an increase in the proportion of hydrogen and carbon monoxide.
At the bottom end of the endothermic entrained bed reactor, the gas is deflected and leaves the apparatus together with the unconverted part of the coke. The gas is subsequently cooled by indirect thermal dissipation and is then passed to the subsequent process stages.
To avoid coke separating off from the gas stream, the speed of the gas is preferably always greater than the rate of suspension of the coke particles, particularly at the deflection site of the gas in the reactor and in the part that may be streaming upwardly.
With this method of carrying out the process and the small grain size of the coke dust, the relative speed between the coke and gas is low, and the residence time of the coke is largely determined by the residence time of the gas, which in turn depends on the extent of the endothermic reactor.
The endothermic gasification of solid carbon with steam and carbon dioxide is a process influenced by the reaction kinetics. The rate of conversion of the solid carbon decreases with a decreasing temperature and increasing proportions of carbon monoxide and hydrogen formed. For this reason, an insufficient relative speed between the solid carbon and the gas and too short of a residence time of the carbon and the gas in the reactor is should be considered as the primary causes of the carbon conversion being too low. As a result of the small grain size and the low relative speed between the solid carbon and the gas, the residence time is not controllable in the case of executing the process according to DE 197 47 324, and it is extendable only by enlarging the reactor.
In the case of stationary fluid bed gasification, the gasification medium streams upwardly from the bottom toward the top, against the gravity. The reactor cross-section is dimensioned in such a way that the gas speed is below the rate of suspension of the fuel grains being used. As a result, an excess of fuel is always present in the reactor, in comparison with the gasification medium used and the converted fuel, guaranteeing a high conversion of the fuel.
In the case of the non-stationary fluid bed, the speed of the gas is higher than the suspension rate of the fuel grains. In this case, the required fuel conversion is achieved by recycling the non-converted part of the fuel into the reaction zone of the reactor.
In the case of the stationary and non-stationary fluid bed gasification of fuels containing proportions of volatiles, it occurs that tars and relatively large proportions of methane and other hydrocarbons are always contained in the gas, as a result of the drying, degasification and gasification processes that are taking place in parallel in the reactor. The tars need to be removed from the gas before it can be utilized for syntheses, but also in the case that the generated gas is to be utilized for energy purposes, e.g., in gas engines. This leads to high expenditure levels in gas purification and gas effluent treatment.
Other hydrocarbons, such as, e.g., methane, are not gas components that can be synthesized. They are consequently undesirable substances present in the gas and reduce the effectiveness of the synthesis.